The present invention relates to internal combustion engines and, more particularly, to method and apparatus for operating internal combustion engines in a thermally efficient manner.
The predominant internal combustion engine configuration in present use is the crankshaft/cylinder arrangement with a working piston reciprocating within a cylinder and connected to a rotatable crankshaft. The piston reciprocates within its cylinder in accordance with one of two predominant operating cycles, namely, the two- or four-stroke spark-ignition Otto cycle or the two- or four-stroke compression-ignition Diesel cycle. With the spark-ignition cycle, a homogeneous mixture of air and fuel at a preferred air/fuel ratio is compressed with ignition caused by an electrical spark or the equivalent. In the compression-ignition cycle, fuel is injected into air that has been compressed to cause an adiabatic increase in its temperature to a temperature above the auto- or self-ignition temperature of the fuel.
Both types of operating cycles and the various physical engine configurations that have been developed have proved satisfactory although each has attendant drawbacks. In the spark-ignition engine, the fuel must be pre-mixed with air to provide a desirably homogeneous mixture with the ratio of the air to the fuel controlled so as to fall within a preferred ratio range, e.g., between 11:1 and 17:1. Air/fuel ratios greater than 17:1 result in mixtures which may or may not combust and air/fuel ratios below 11:1 result in mixtures which are inefficient from the standpoint of fuel consumption and unacceptable with regard to air pollution. Additionally, the compression ratio of the spark-ignition engine is limited to some maximum so as not to cause unintentional pre-ignition of the homogeneous air/fuel mixture during the compression stroke. The compression ratio limit also disadvantageously limits the thermal efficiency of spark-ignition engines.
In contrast to the spark-ignition engine, the compression-ignition engine utilizes air that has been heated during the compression stroke to a temperature greater than the auto-ignition temperature of the fuel so that fuel can be injected in a heterogeneous manner into the so-heated air to cause burning. Thus, the fuel injected in a compression-ignition engine can be burned in considerable excess air to provide a comparatively large mass of heated air for the expansion stroke. Accordingly, the compression-ignition engine provides a substantial increase in thermal efficiency compared to the spark-ignition engine. Unfortunately, compression-ignition engines require a rather sophisticated and expensive fuel delivery and injection system that mitigates against the increase in thermal efficiency.
A third combustion cycle, the Gerace cycle as disclosed in applicant's U.S. Pat. Nos. 4,520,765 and 4,635,590, achieves a synthesis of the Otto and Diesel cycles by providing a piston/cylinder chamber that serves as a low pressure air-fuel chamber and a second piston/cylinder chamber that serves as a high pressure, high temperature ignition-air chamber. A valve separates the two chambers and is opened and closed through the cycle to effect operation. Prior to the compression cycle, the valve is closed and an air-fuel mixture is compressed to a selected compression ratio less than that which would cause pre-ignition. Concurrently, the air in the ignition-air chamber is compressed to a selected compression ratio that is sufficient to raise the temperature substantially above the ignition temperature of the air-fuel mixture in the air-fuel chamber. Ignition is achieved by opening the valve to permit the high pressure, ignition temperature air to discharge into the air-fuel cylinder. The ignition air is quickly vented into the heated air-fuel mixture, creating turbulent combustion, and mass ignition of the fuel. The rapidly increasing pressure from the burning air-fuel mixture in the air-fuel chamber is transferred through the still open valve to the ignition-air chamber which provides additional oxygen to completely oxidize any unburned fuel. As a consequence, the homogeneous air-fuel charge is burned under excess air conditions in which a relatively large mass of heated gas and combustion products operates against the working pistons. The net result is the complete burning of the fuel at the very beginning of the expansion cycle providing high pressure gas for the expansion process.